A navigation system has a route guide function, in which a route guide is performed when a subject vehicle approaches a guide-target intersection (or guide-target branch point), at which the vehicle needs to turn right or left in order to follow a guide route to a destination. For instance, an enlarged guide image around the intersection is displayed in a screen window on a display unit and, furthermore, an audio guide, e.g., “turn left xx meters ahead” is sounded.
Patent document 1 proposes a navigation system to appropriately perform a route guide for a user at a guide-target branch point. This navigation system performs a route guide each time a subject vehicle approaches a guide-target branch point. The navigation system memorizes alert information relating to both a certain guide-target branch point and its surroundings on the guide route when the vehicle deviates from the certain guide route.
The navigation system afterward outputs an alert based on the memorized alert information when the vehicle approaches the same certain guide-target branch point on the same guide route. This helps prevent a user of the vehicle from deviating again from the guide route at the certain guide-target branch point.                Patent Document 1: JP-2000-9484 A        
In contrast, the navigation system in Patent document 1 outputs no alert when the vehicle does not enter the same certain guide-target branch point by following the same guide route. The same user does not frequently mistake a guide route at the same branch point. Therefore, the navigation system in Patent document 1 may not effectively prevent a user from deviating from a guide route using the alert outputted.
Another method may be used to enhance effectiveness. For instance, when a certain intersection has a tendency to cause a user to mistake a guide route, the certain intersection is previously designated. An alert is thereafter outputted only when a subject vehicle approaches the certain intersection designated. This method may still bother a user who hardly mistakes a guide route.